Reasons cited were:
Huge library of add-on modules to extend functionality very easily.
More secure than proprietary software because of large community working to find security concerns.
Ability to keep the current design and functionality while rebuilding everything behind the scenes.
Ability to address their specific needs.

Many interesting reactions to this news:

Exciting news! Whitehouse.gov has been relaunched in Drupal!

Reasons sited were:
Huge library of add-on modules to extend functionality very easily.
More secure than proprietary software because of large community working to find security concerns.
Ability to keep the current design and functionality while rebuilding everything behind the scenes.
Ability to address their specific needs.